1. Field of the Invention
The present invention relates to honeycomb structural bodies to be used for purifying exhaust gas emitted from an internal combustion engines, and electrically heated catalyst (EHC) devices equipped with the honeycomb structural body.
2. Description of the Related Art
In general, a catalyst device is mounted to an exhaust gas pipe of an exhaust gas purifying system through which exhaust gas emitted from an internal combustion engine of a motor vehicle flows and purified. The catalyst device is capable of purifying exhaust gas. In general, such a catalyst device uses a honeycomb structural body which supports catalyst such as platinum (Pt), palladium (Pd), rhodium (Rh), etc., therein.
By the way, it is necessary to heat the catalyst supported in the catalyst device at approximately 400° C. in order to adequately activate the catalyst. In order to achieve this, there is a conventional technique, an electrically heated catalyst (EHC) device having a honeycomb structural body equipped with a pair of a positive electrode and a negative electrode. The positive and negative electrodes are formed on the outer peripheral surface of the honeycomb structural body. When electrical power is supplied to the positive and negative electrodes, current flows in the honeycomb structural body, and heat energy is generated in the honeycomb structural body.
For example, Japanese patent laid open publication No. 2010-106735 has disclosed a honeycomb structural body having a cylindrical shape on which a pair of a positive electrode and a negative electrode is formed on the outer circumferential surface thereof. When a honeycomb structural body has a cylindrical shape (namely, whose cross section perpendicular to an axial direction of the honeycomb structural body has a circular shape), a distance between the positive electrode and the negative electrode forming the electrode pair is changed according to a portion of the electrodes, namely, not constant. This electrode structure causes a problem of it being difficult for the current to flow through a long distance between the electrode pair (for example, a distance at the central parts between the positive electrode and the negative electrode) when compared with the current flowing through a short distance at the end parts between the positive electrode and the negative electrode (for example, a distance at the outside portion between the positive electrode and the negative electrode). This causes non-uniform temperature distribution in the inside of the honeycomb structural body because of causing unbalanced temperature distribution in the inside of the honeycomb structural body.
In order to solve the above conventional problem, there is a conventional technique, disclosed in Japanese patent laid open publication No. H04-67588. This conventional technique provides a honeycomb structural body of a cylindrical shape having a heater part and a slit part. The slit part is formed in the honeycomb structural body and adjusts the electrical resistance in the honeycomb structural body.
Further, there is other conventional technique, disclosed in Japanese patent laid open publication No. H04-280086, which provides a honeycomb structural body equipped with a honeycomb monolith heater. In particular, a cross section of the honeycomb structural body, which is perpendicular to an axial direction of the honeycomb structural body, has a square shape or a race-track shape in order to have the same distance between the electrode pair.
However, the honeycomb structural body with the electrodes disclosed in Japanese patent laid open publication No. H04-67588 has a long electrical path because of having the slit part in order to adjust the electrical resistance. This structure causes a difficulty of increasing the temperature of the honeycomb structural body as soon as electrical power is supplied.
Further, this conventional honeycomb structural body has a possibility of decreasing the entire strength and the capability of purifying exhaust gas because of having the slit part in the honeycomb structural body.
Because the honeycomb structural body with the honeycomb monolith heater disclosed in Japanese patent laid open publication No. H04-280086 has a square shape or a race-track shape, there is a possibility of it being difficult to mount it in an exhaust gas pipe of a motor vehicle. That is, the structure of this conventional honeycomb structural body has a drawback on mounting it to the exhaust gas pipe in the exhaust gas purifying system for the internal combustion engine of a motor vehicle.